1. Field of the Invention
The present invention is in the field of plastic films or sheets and more particularly relates to porous or liquid permeable thermoplastic films.
Porous or perforated thermoplastic films have many useful applications. Such films are useful in gardening and farming to prevent the growth of grass or weeds while permitting moisture to be transmitted through the film to the solid beneath. They are used for making disposable diapers and other various absorbent structures and for packaging of foods and other materials.
The invention also relates to absorptive structures made from the porous film, such as diapers, sanitary napkins, bed pads, incontinent pads, towels, bandages and the like. The invention particularly relates to porous film used as topsheets for such structures.
The invention especially relates to an improved porous film or topsheet which allows fluid to pass to the interior of the absorptive device but which inhibits the reverse flow of the fluid. In general, the topsheet is the portion of an absorptive device which covers one face of the absorbent element of the absorptive structure and which in some applications contacts the skin of a person using the absorptive device.
2. Description of the Prior Art
Particularly useful absorptive devices are articles of manufacture designed to receive and retain fluid discharges from the body within an absorbent element of the absorptive device. Absorptive devices such as sanitary napkins, catamenial tampons, bed pads, incontinent pads, towels, bandages and the like are well known articles of commerce. In recent times, single use disposable absorptive devices have significantly replaced permanent absorptive devices which were designed to be laundered and reused. While the improved absorptive structure of this invention can be used with reusable absorptive devices, it finds great utility when used with disposable absorptive devices.
Perforated thermoplastic films of polyethylene, polypropylene, polybutene-1, polyvinyl chloride, and other flexible thermoplastics normally extruded into such films or thin sheets have been made by various methods. One method is to extrude the thermoplastic material, e.g., polyethylene, from a conventional slot extrusion die onto a continuously moving, smooth, cooled casting surface, e.g., a chill roll. A pattern may be applied to the chill roll and the film pressed to the roll while in the amorphous or molten stage by press rolls. Alternatively, the chill roll may be very smooth and a desired pattern in the film may be mechanically impressed into the film on the chill roll by pressing the roll against the film and the chill roll to impress the pattern into the film as it is cooled on the chill roll. The softness of the film produced by chill casting is directly related to the density of the polyethylene resin used. In order to obtain different degrees of softness or stiffness, it is necessary to use a number of polyethylene resins having different densities. Thus, if it is desired to produce a relatively stiff embossed film, it is necessary to use more expensive polymers having high densities as the feed material to the slot die.
Film rolls of poor conformation produce problems when running the film through fabricating machines or through a film printing apparatus.
An example of a method and apparatus for producing film according to the foregoing slot die-chill cast roll technique is shown in U.S. Pat. No. 3,374,303.
Another technique used for making plastic film has been the utilization of a heated engraved embossing roll in conjunction with a backup roll. The preformed strip of thermoplastic film normally at room temperature, is passed between the nip of a heated engraved roll and a backup roll and is embossed by being heated while in contact with the heated, engraved roller. The resultant embossed film usually has a very shallow and poorly defined pattern. An example of an apparatus and process for carrying out a process of this type is shown in U.S. Pat. No. 3,176,058.
Still another process for making thermoplastic film has been to pass the film over a heated roll or a series of heated rollers in order to heat the film to a softened state and then to contact the film with an embossing roller and to press the film against the embossing roller by a backup roller. Normally, the embossing roller and the backup rollers are cooled in order to freeze the pattern into the film so that it may be immediately wound up into rolls, if desired. An apparatus and process for preparing an embossed film according to the foregoing is shown in U.S. Pat. No. 3,246,365.
A more recent process for making plastic material is shown in U.S. Pat. No. 3,950,480, wherein the film is heated by a non-direct contact heat source to raise the temperature of the film above its softening point and the film is then immediately fed between adjacent, counter-rotating rollers, and thereby embossed.
A method for perforating thermoplastic sheet or film is disclosed in U.S. Pat. No. 3,054,148, issued to Zimmerli, which reference is hereby incorporated herein. The Zimmerli patent discloses a stationary drum having a molding element mounted around the outer surface of the drum which is adapted to rotate freely thereon. A vacuum chamber is employed beneath the screen or molding element to create a pressure differential between the respective surfaces of the thermoplastic sheet to cause the plasticized sheet to flow into the perforations provided in the molding element and thereby cause a series of holes to be formed in the sheet.
U.S. Pat. No. 4,155,693 and U.S. Pat. No. 4,157,237 illustrate types of screens or molding elements.
U.S. Pat. No. 4,252,516 and U.S. Pat. No. 4,317,792 disclose apparatus and method, respectively, for manufacturing thermoplastic sheet having elliptical holes.
Disposable absorptive devices comprising an absorbent pad covered with a topsheet which contacts the body are well known. Covering the outer portion of the absorptive device with a fluid-impermeable backsheet to prevent absorbed fluids from leaking out of the absorptive device and soiling clothing, bed clothes, etc. is equally well known. The absorbent pad component of disposable absorptive devices can comprise well known materials such as creped cellulose wadding, airlaid felt or the like. The liquid impermeable backsheet can comprise any of various materials well known in the art such as polyethylene film.
One of the principle disadvantages of conventional absorptive devices is the maceration of the skin caused by prolonged contact with absorbed fluids. One especially common manifestation of this maceration is diaper rash generally occurring about the base of the trunk of infants. In order to minimize the effect of prolonged liquid contact with the skin, absorptive devices such as diapers have been produced with the body contacting topsheet thereof designed to exhibit a greater or lesser degree of surface dryness. For example, U.S. Pat. No. 3,327,625 issued to Johnson on Mar. 1, 1966, teaches that any hydrophobic material in the crotch area of the diaper will cause moisture to wick away from the skin of an infant wearer and thereby provide a substantially dry surface in contact with the infant's skin. U.S. Pat. No. Re. 26,151 issued to Duncan et al. on Jan. 31, 1967 teaches the use of porous, hydrophobic, nonwoven fabrics as topsheets. U.S. Pat. No. 2,916,037 issued to Hansen on Dec. 8, 1959, is a further example of the use of a nonwoven topsheet.
U.S. Pat. No. 3,814,101 issued to Kozak on June 4, 1974 illustrates still another type of disposable absorbent article. Such patent discloses a topsheet of non-fibrous hydrophobic film which has a plurality of valvular openings or slits therein and a system of depressed areas disposed across the surface of the topsheet. The openings permit the flow of liquid in one direction of the absorbent but reduce the flow of the liquid in the opposite direction.
U.S. Pat. No. 3,989,867 which issued to Sisson on Nov. 2, 1976, describes a breathable liquid impervious backsheet containing apertured bosses. The apertures therein, in order to maintain the liquid impervious character of the backsheet, are smaller in diameter than the capillaries of U.S. Pat. No. 3,929,135 hereinafter described.
U.S. Pat. No. 3,929,135 issued to Thompson on Dec. 30, 1975, relates to absorptive devices utilized a topsheet having tapered capillaries of critical diameters and tapers which allow fluid to pass into the interior of an absorptive device and which inhibit the reverse flow of such fluid.